The present invention relates to a microwave bandpass filter adapted to use as a frequency converter and the like and, more particularly, to an improvement in a stub type bandpass filter which exhibits substantial attenuation to the waves which are double or treble its passband.
Stub type bandpass filters have been known in which a branch line or stub is associated with a transmission line such as a strip line, a microstrip line, wave guide and coaxial cable, to furnish is with filtering characteristics. One type of such bandpass filters has two stubs associated with a 1/4 wavelength transmission line through which a signal is passed. This type of filters is generally classified into three kinds, i.e., a first filter in which the ends of both stubs are open, a second filter in which the ends of both stubs are short-circuited, and a third filter in which the end of one stub is open and the end of the other is short-circuited. In the second or third kind of filter, the stubs resonate responsive to the waves which are the integral multiples of the fundamental passband. It follows that the filter passes the waves which are the integral multiples of the passband as well. The third kind of filter passes the waves which are the odd multiples of the passband.
Thus, the prior art filter having two stubs connected with a 1/4 wavelength transmission line passes the integral multiple waves of the passband. When such a filter is applied to a frequency converter or a mixer, it is impossible to confine higher-order harmonic signals in a mixer diode and, therefore, to reduce the conversion loss.